The following disclosure relates to electrical circuits and signal processing.
A wireless communications transmitter typically converts an information-bearing baseband signal from around DC to a high frequency referred to as the carrier frequency (e.g., a frequency in the microwave or RF band) that is suitable for wireless transmission. In many systems, this frequency upconversion process takes place in multiple stages. The baseband signal is first upconverted to an intermediate frequency (fIF) that is higher than the bandwidth of the baseband signal. At the intermediate frequency, the signal is amplified and filtered before the signal is upconverted to the carrier frequency (fc) for transmission.
In an ideal transmitter, all transmitted signal energy is confined to a dedicated frequency channel, and no energy is emitted outside the channel to interfere with other wireless systems. In practical realizations, out-of-band spurious emissions often are generated by transmitters due to, for example, local-oscillator (LO) harmonics, image generation, and intermodulation.
Spurious emissions caused by the mechanisms mentioned above can fall into restricted frequency bands and result in an emissions violation. Conventional transceivers can use highly selective external filters (e.g., SAW filters) to suppress undesirable spurious emissions. External filters add to the overall cost and size of the transceiver.